I have BD139/140 on the working side and have ordered several from CPC but had to order the Mj2955 from Cricklewood not stocked at CPC.
Never easy working off images 🙂 but it looks reasonable. 0.25 watt is a bit low (can you even get 0.22 ohm in that wattage). 2 watt is fine for something like the NAD.
Original parts arrived and fitted today replace Q414 with BD140 and Q412 & Q408 with BD139's. Doubled checked every thing for correct orientation and powered on with the DBT, build flashed once and we have a green light on the front and no magic smoke yet.
Tested for voltage on both speaker output a little high but present, connected some old speakers and could hear a crackle from both speaker when rotating the balance/volume control.
Feeling confident I connected my PC to CD input but no sound from the speakers, checked the speaker and input selector switches are in the correct position but no sound on any of the inputs?
I could smell something getting hot, powered off and found Q501 669a was getting hot, removed and it tested OK, also removed C508 & C507 and tested Ok at 325uf 0.8 ohms and 328uf 0.9 ohms but C512 & C511 read 39uf 1.8 ohms & 33uf 2.1 ohms.
Obviously, C512 & C511 need to be replaced, however, this looks like the power supply to me and not the audio input area that appears to be the current problem?
Tested for voltage on both speaker output a little high but present, connected some old speakers and could hear a crackle from both speaker when rotating the balance/volume control.
Feeling confident I connected my PC to CD input but no sound from the speakers, checked the speaker and input selector switches are in the correct position but no sound on any of the inputs?
I could smell something getting hot, powered off and found Q501 669a was getting hot, removed and it tested OK, also removed C508 & C507 and tested Ok at 325uf 0.8 ohms and 328uf 0.9 ohms but C512 & C511 read 39uf 1.8 ohms & 33uf 2.1 ohms.
Obviously, C512 & C511 need to be replaced, however, this looks like the power supply to me and not the audio input area that appears to be the current problem?
Q501 is a series regulator which means the heat produced depends on the product of the current through the transistor and the voltage dropped across it. I would check the voltage across R447 and R448. No supply voltages on the diagram as such but I would expect around 1 volt give or take across each of those resistors.
Did you do what I said and check those joints on the regulated supply ,you won't get sound if they loose or blown. All fuses OK? they will get hot if the connections are loose. Pre amp connection bars in at the back?
Yes while waiting for parts to arrive, I went over the boards looking for dry joint and soldered any questionable joints including the regulated supply and all fuses are intact, pre amp connection installed. I will test R447 and R448 but don't see any voltages on the service Manual Schematic what voltages should I expect to see at key points.
I have removed them to test due to age and being linked to Q501 getting hot and the previous repairer had scratch + & - on the solder resist side on both capacitors.
were both of these output transistors definatly faulty?? its very rare for both to be blown
i know you said you checked around the board for dry joints, but look again at those 2 regulators, just to be sure at, and rock them back and forth gently to see if they are loose on the board.
The PNP will be the green one(if original) and this will be discoloured with heat if loose(replace it D649a)
I have repaired dozens of these and the most common fault is one blown output+ the driver
Other components to check are
R649
R447
Q501
Q502
Q503
Q504
R501
R506
Make sure when you remove RX1/2 you dont accidently remove the one next to it,it is easily done(R453/4) this will thow the current way out and it will get hot very quickly
replace vr401/2 as they are not very good (if originals) and dont adjust properly most of the time-as @Mooly would say,they get very flakey
ive attached a realy good scanned copy of the scheme for you in case you have one of the poorer versions
cheers
paul
Thanks for all the information above and the better quality schematic, I have refitted the caps I removed yesterday as I had some in stock and a new Q501 then turned on but still no input and Q501 was still getting hot.
I then referred to your list of components to test and found and working through the list I found that Q503 A1015GR was dead, This was replaced and powered on......Q501 was not over heating and when I connected the input to my computer and much to my delight sound on both channels🙂.
I then connected my meters to the speaker outputs and as you said the variable resistors are flakey VR401 was Ok but VR402 would not move and had to replace it, eventually all adjusted well (photo Attached). I needed to set the idling current but need to replace VR401/402 before I attempt this!
I would like to say a big thank you to especially Mooly and Poundy for all your time help given with this repair, your help has been invaluable.
Thanks again
Regards
David
Next project Akia 4000DS service😉
I then referred to your list of components to test and found and working through the list I found that Q503 A1015GR was dead, This was replaced and powered on......Q501 was not over heating and when I connected the input to my computer and much to my delight sound on both channels🙂.
I then connected my meters to the speaker outputs and as you said the variable resistors are flakey VR401 was Ok but VR402 would not move and had to replace it, eventually all adjusted well (photo Attached). I needed to set the idling current but need to replace VR401/402 before I attempt this!
I would like to say a big thank you to especially Mooly and Poundy for all your time help given with this repair, your help has been invaluable.
Thanks again
Regards
David
Next project Akia 4000DS service😉
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